SELECTIVE PROTONATION OF TERMINALLY BOUND ETA-3-ALLYLS ON (ETA-3-ALLYL)2(MU-2-ETA-3-ALLYL)2DIMOLYBDENUM(II) BY BETA-DIKETONES FORMING (MU-2-ETA-3-ALLYL)2(ETA-2-BETA-DIKETONATE)2 DIMOLYBDENUM(II) COMPLEXES

The beta-diketones, R(C = O)CH2(C = O)R' [where R = R' = Me (acac); R = Me, R' = CF3 (tfac); or R = R' = CF3 (hfac)] selectively protonate the terminally bound eta-3-allyls on Mo2(mu-2-eta-3-allyl)2(eta-3-allyl)2 (A) leaving the Mo2(mu-2-eta-3-allyl)2 core intact. The molybdenum(II) dimers, Mo2(mu-2-eta-3-allyl)2(eta-2-L)2 [where L = acac (B), tfac (C) or hfac (D)] are formed from this reaction. From H-1, C-13 and F-19 NMR data, it is evident that the two bridging allyls in each complex, B-D, are bound in a cis arrangement with endo and exo conformations, respectively. Thus, a plane of symmetry bisects the Mo-Mo bond axis and the two bridging allyls in B and D, but is absent parallel to the Mo-Mo bond axis and bisecting each of the chelating beta-diketonate ligands. The complex, C, exists primarily as the isomer in which the bulky trifluoromethyl groups on the two tfac ligands are as far apart as possible from each other. As the electron-donating ability of the terminally bound ligands in A-D decreases, allyl > acac > tfac > hfac, an upfield shift in the proton resonances of the bridging allyls is observed.